22 Design and Optimization of Thermal Systems It is important to recognize that thermal systems arise in many diverse fields of engineering, such as aerospace engineering, manufacturing, power generation, and air conditioning Consequently, a study of thermal systems usually brings in many additional mechanisms and considerations, making the problem much more complicated than what might be expected from a study of thermal sciences alone 1.3.2 ANALYSIS The analysis of thermal systems is often complicated because of the complex nature of fluid flow and of heat and mass transfer mechanisms that govern these systems As a result, typical thermal systems have to be approximated, simplified, and idealized in order to make it possible to analyze them and thus obtain the inputs needed for design Following are some of the characteristics that are commonly encountered in thermal systems and processes: 10 11 12 Time-dependent Multidimensional Nonlinear mechanisms Complex geometries Complicated boundary conditions Coupled transport phenomena Turbulent flow Change in phase and material structure Energy losses and irreversibility Variable material properties Influence of ambient conditions Variety of energy sources Because of the time-dependent, multidimensional nature of typical systems, the governing equations are generally a set of partial differential equations, with nonlinearity arising due to convection of momentum in the flow, variable properties, and radiative transport However, approximations and idealizations are used to simplify these equations, resulting in algebraic and ordinary differential equations for many practical situations and relatively simpler partial differential equations for others These considerations are discussed in Chapter as part of modeling of the system However, the equations for a few simple cases are given here to illustrate the nature of the governing equations and the effect of some of these complexities The simplest problems are those that assume steady-state conditions, with or without flow, while also assuming uniform conditions in each part of the system These problems lead to algebraic equations, which are often nonlinear for thermal systems This situation is commonly encountered in thermodynamic systems such as refrigeration, air conditioning, and energy conversion systems Then, the governing set of algebraic equations may be written as f1 (x1, x2, x3, , xn) f (x1, x2, x3, , xn) Introduction 23 f (x1, x2, x3, , xn) f n (x1, x2, x3, , xn) (1.4) where the xi are the unknowns and the functions fi, for i 1, 2, 3, , n, may be linear or nonlinear Such problems are generally referred to as steady, lumped circumstances and have been most extensively treated in papers and books dealing with system design, such as Hodge (1985), Stoecker (1989), and Janna (1993) However, these approximations are applicable for only a few idealized thermal systems Additional complexities, mentioned earlier, generally demand analysis that is more accurate and the solution of ordinary and partial differential equations Nevertheless, because of the ease in analysis, steady lumped systems are effective in illustrating the basic ideas of system simulation and design Therefore, these are used as examples throughout the book while bearing in mind that, in actual practice, more elaborate analysis would generally be needed If the time-dependent behavior of the system is sought, for a study of the dynamic characteristics of the system, the resulting governing equations are ordinary differential equations in time, if the assumption of uniform conditions within each part is still employed Then the governing equations may be written as dxi d Fi ( x1 , x , x3 , , xn ) for i 1, 2, 3, ,n (1.5) where, again, the functions Fi may be linear or nonlinear The systems for which these approximations can be made are known as dynamic, lumped systems Such a treatment is valuable in many cases because of the resulting simplicity In many thermodynamic systems, such as heat engines and cooling systems, the individual components are approximated as lumped and the dynamic analysis is of interest in the startup and shutdown of the systems, as well as in determining the effects of changes in operating conditions like flow rate, pressure, and heat input If the conditions in the different parts of the system cannot be assumed to be uniform, the problem is referred to as distributed A time-dependent, twodimensional flow with the assumption of constant fluid properties, as in a duct or over a heated body, is represented by the equations (Burmeister, 1993) u x u v v y (1.6) u u x v u y p x u x2 u y2 (1.7) u v x v v y p y v x2 v y2 (1.8) 2 2 where the first equation gives the conservation of mass and the other two give the momentum force balance in the x and y directions, respectively Here, u, v are the 24 Design and Optimization of Thermal Systems velocity components in the x, y coordinate directions, p is pressure, is the density, is time, and is the kinematic viscosity of the fluid The corresponding energy equation for the temperature T in the fluid, with as its thermal diffusivity, is T u T x v 2 T x2 T y T y2 (1.9) The corresponding three-dimensional equations may similarly be written by adding the components in the z direction, including the z-momentum equation The problem then becomes much more complicated, but many practical circumstances, such as environmental processes, cooling of electronic equipment, and manufacturing systems, require three-dimensional analysis for accurate results for use in design and optimization If there is no flow, for instance, in the circumstance of conduction in a stationary solid body such as the wall of a building or of a blast furnace, the energy equation, Equation (1.9), reduces to T x2 T y2 T (1.10) Here, the energy equations, (1.9) and (1.10), are linear because T appears in its first power The flow is obtained from Equation (1.6) through Equation (1.8), which are coupled with u, v, and p as the unknowns The momentum equations are nonlinear because of the inertia terms u ∂u/∂x, v ∂u/∂y, etc., in which higher powers of the unknown velocity components appear Equation (1.6) through Equation (1.10) are partial differential equations and are written for the relatively simpler constant property, two-dimensional circumstance for the Cartesian coordinate system and a Newtonian fluid Even then, these are quite complicated In practical systems, we often encounter many additional complexities that make the analysis a very difficult and challenging affair Inclusion of variable properties and/or radiative transport can give rise to nonlinear mechanisms, the former due to the dependence of the properties on the dependent variable such as temperature T and the latter due to the variation of radiation heat transfer as T For example, if the thermal conductivity k, density , and specific heat at constant pressure Cp vary with temperature T, Equation (1.10) becomes (T )C p (T ) T x k (T ) T x y k (T ) T y (1.11) Similarly, Equation (1.9) may be modified for variable properties Thus, nonlinearity arises due to nonlinear powers of T resulting from property variation with T If radiative heat loss occurs at a surface, the corresponding energy exchange rate q, per unit area, with a black environment at temperature Te may be written as q (T Te4 ) (1.12) Introduction 25 where is the surface emissivity and is the Stefan–Boltzmann constant This equation is nonlinear in T due to the presence of temperature as T Thus, nonlinear equations are frequently obtained, making the solution difficult Iterative methods are often needed to obtain the solution Nonlinearity also makes it difficult to scale up the results from a laboratory model to the full-size system Many of these considerations are discussed in detail in Chapter The various other complexities mentioned earlier also complicate the analysis and design of thermal systems Complex geometry and boundary conditions arise in most practical systems, making it necessary to use simplifications and versatile numerical techniques such as finite element and boundary element methods Turbulent flow is encountered in many important processes, particularly in energy systems and environmental transport Special numerical models and experimental procedures have been developed to take turbulent transport into account Phase change, coupling with material characteristics, time-varying ambient conditions, irreversibility, and different energy sources, such as lasers, gas, oil, electricity, and viscous heating, further complicate the analysis of thermal systems and processes Several of these aspects will be seen to arise in examples given in later chapters However, our focus is not on analysis but on design, even though analysis provides many of the inputs needed for design Therefore, only a brief outline of the basic characteristics of thermal systems is given here Specialized books, such as Ozisik (1985) and Incropera and Dewitt (2001) in heat transfer, Fox and McDonald (2003) and Shames (1992) in fluid mechanics, Howell and Buckius (1992), Cengel and Boles (2002) and Moran and Shapiro (2000) in thermodynamics, among others, may be consulted for details on different analytical and experimental techniques as well as for results obtained on a variety of fundamental and applied problems 1.3.3 TYPES AND EXAMPLES As mentioned earlier, thermal systems are important in a wide variety of engineering fields and disciplines Let us consider some important examples, types, and applications of these systems Several different ways of classifying thermal systems may be employed because of their diversity A common method is in terms of the function or application of the system Using this approach, several important types of thermal systems, along with commonly encountered applications and examples, follow Manufacturing and Materials Processing Systems Examples include processes such as casting, crystal growing, heat treatment, metal forming, drying, soldering and welding, laser and gas cutting, plastic extrusion and injection molding, powder metallurgy, optical fiber drawing, ceramics, and glass processing Also included are food processing systems as well as common household appliances such as ovens and cooking ranges This is an important area and many diverse thermal systems are employed for the different manufacturing processes used in practice We have already discussed ingot casting, as 26 Design and Optimization of Thermal Systems Tundish Liquid metal feed y Water cooled mold δ(Solidified skin thickness) Water sprays + + + + Supporting and withdrawing rolls Withdrawal of casting at velocity U U (a) Feed hopper Heated and cooled surface Extrudate Die (b) FIGURE 1.10 A few manufacturing systems (a) Continuous casting, (b) plastic screw extrusion, (c) optical fiber drawing, (d) hot rolling (Figure 1.10(a) adapted from Ghosh and Mallik, 1986; Figure 1.10(b) from Tadmor and Gogos, 1979.) sketched in Figure 1.3 Figure 1.10 shows the schematics for continuous casting, plastic extrusion, glass fiber drawing, and hot rolling processes In continuous casting, molten material is allowed to solidify across an interface as the bulk material is withdrawn at a given speed through a mold, which is Introduction 27 Feed mechanism Glass rod (preform) Furnace Furnace Neckdown region Fiber diameter monitor Accelerated cooling Fiber cooling distance Fiber Coating applicator Coating concentricity monitor Curing furnace or lamps Coating diameter monitor Coated fiber Drawing pulley Winding drum (c) Station Station Hot material U Temperature Rollers Distance (d) FIGURE 1.10 (Continued) 28 Design and Optimization of Thermal Systems usually water cooled In plastic screw extrusion, the solid plastic is fed through a hopper, melted by energy input, and conveyed by the rotation of the screw, with an associated pressure and temperature rise, finally pushing the molten material through a die to obtain a desired shape The material may also be injected into a mold and solidified for a process known as injection molding Similarly, a special glass rod, typically 5–10 cm in diameter and known as a preform, is heated in a furnace and pulled to sharply reduce the diameter to 100–125 m, yielding an optical fiber in glass fiber drawing In hot rolling, the material is heated and reduced in thickness by pushing it through two rollers that are at a given distance apart Several sets of rollers may be used to obtain the desired decrease in thickness or diameter Similarly, new thermal processes have been developed for the fabrication of nanomaterials through chemical vapor deposition and other approaches Heat transfer is very important in these processes because the temperature determines the forces needed, the withdrawal speed, and the quality of the final product Further details on these and other processes may be found in specialized books on manufacturing, such as Ghosh and Mallik (1986), Doyle et al (1985), and Kalpakjian (1989) Some of these processes will be considered again later in the book as examples With the development of new and improved materials, the design of thermal systems for materials processing has become crucial for manufacturing new products and for meeting international competition Energy Systems Examples of energy systems include power plants, solar energy utilization, geothermal energy systems, energy storage, solar ponds, and conventional and nonconventional energy conversion systems This is one of the most frequently mentioned areas for thermal energy considerations Different types of thermal systems arise depending on the nature of the energy source, such as nuclear, oil, gas, solar, or wind energy Most of these systems are covered in thermodynamics courses and are often treated as steady, lumped systems Figure 1.11 shows sketches of typical solar and nuclear energy systems In both cases, the energy collected or generated is used to run the turbines, which are then used to generate electricity A considerable literature exists on thermal systems of interest in this field because of the tremendous importance of power generation in our society; see, for instance, Howell et al (1982), Hsieh (1986), Van Wylen et al (1994), and Duffie and Beckman (1991) Cooling Systems for Electronic Equipment Systems that are of interest in this area include air cooling, liquid immersion, heat pipes, heat sinks, heat removal by boiling, and microscale systems This is one of those areas where thermal considerations are extremely important for the satisfactory performance of the system even though the main application is in a different area Thus, electronic systems, such as computers, televisions, digital multimeters, and signal conditioners, are used for a variety of applications, Introduction 29 Solar boiler Solar flux Bypass line Power output Turbine Storage Pump Solar energy collection system Condenser (a) Containment vessel Steam generator Nuclear reactor Highpressure turbine Lowpressure turbine Power output Condenser Primary loop pump Secondary loop pump (b) FIGURE 1.11 Power systems based on (a) solar energy and (b) nuclear energy (Adapted from Howell and Buckius, 1992.) 30 Design and Optimization of Thermal Systems most of which are not directly connected with fluid flow and heat transfer But the cooling of the electronic system in order to ensure that the temperature Tc of the various components, particularly of the chips or semiconductor devices, does not exceed the allowable temperature level Tmax, that is, Tc Tmax, is often the most crucial factor in the design and operation of the system Further size reduction of the system is frequently constrained by the heat transfer considerations Figure 1.12 shows typical air cooling and liquid immersion systems for electronic equipment The energy dissipated by the electronic components is removed by the fluid flow, thus allowing the temperatures to remain below the specified limit Figure 1.2 showed a sketch of a heat pipe for enhanced cooling of an electronic chip Many books, such as those by Steinberg (1980) and Kraus and Bar-Cohen (1983), have been written in response to the growing importance of this area Photographs Air flow Electric circuit boards Wall (a) Coolant out Coolant in Relief valve Expansion chamber Condenser plate Electronic components Electrically nonconducting liquid (b) FIGURE 1.12 Cooling systems for electronic equipment (a) Forced air cooling, (b) liquid immersion cooling Introduction 31 FIGURE 1.13 Typical electronic systems with air cooling by means of a fan (From Steinberg, 1980.) of typical electronic equipment with air cooling by means of a fan or a blower are shown in Figure 1.13, indicating the complexity of such systems in actual practice Environmental and Safety Systems Examples of these systems include arrangements for heat rejection to ambient air and water, control of thermal and air pollution, cooling towers, incinerators, waste disposal, water treatment plants, smoke and temperature control systems, and fire extinguishing systems The growing concern with the environmental impact of waste and energy disposal, including global warming and depletion of the ozone layer, has made it essential to minimize the effect on our environment by developing new and 32 Design and Optimization of Thermal Systems Power plant heat rejection system Cooling tower T Outfall Ts u Pump x Average flow Temperature profile Lake Hot water Air flow Intake water Cooled water Tb Intake (a) (b) FIGURE 1.14 Systems for heat rejection from a power plant (a) Natural lake as a cooling pond, (b) natural draft cooling tower improved methods for disposal Many thermal systems have been developed in response to this need These include systems based on fluids that would substitute refrigerants like CFCs (chlorofluorocarbons) that adversely affect the ozone layer, improved incineration techniques for solid waste disposal, catalytic converters in automobiles to reduce harmful emissions, and scrubbers in power plants to reduce pollutants Figure 1.14 shows sketches of typical heat rejection systems from power plants, employing a lake as a cooling pond in the first case and a natural draft cooling tower in the second The effect on the local environment, in terms of temperature rise, increased flow, and disturbance to natural yearly cycle, is of particular concern in these cases Safety is also a very important consideration Figure 1.15 shows a sketch of a room fire, indicating a hot upper layer containing the toxic and hot combustion products and a relatively Ceiling Stably stratified region Wake Outflow T Walls Inflow Opening Plume Fire Temperature distribution FIGURE 1.15 Flow and temperature due to fire in a room with an opening Introduction 33 cooler and less toxic lower layer that is often safe until flashover occurs when everything in the room catches fire and the room is engulfed in flames Thus, the design of the system, which may be a building, ship, submarine, or airplane, for fire safety is clearly an important element in the overall construction and operation of the system Aerospace Systems Many thermal systems in aerospace applications are of interest here Some of the common ones are gas turbines, rockets, combustors, and cooling systems This has been a particularly important area over the last three decades because of the space program Considerable progress has been made on the various thermal systems and subsystems that are needed Because of the large thrust needed at rocket launch and high cooling rates during reentry, much of the effort in designing efficient systems has been directed at these two stages However, cooling, air conditioning, and electronic and energy systems during orbit, as well as for a space station, have their own requirements and challenges Transportation Systems Most of the relevant systems in this area are thermal in nature These include internal combustion engines such as spark ignition and diesel engines; steam engines; fuel cells; and modern automobile, airplane, and train engines This is an extensive field, closely associated with different kinds of thermal systems Though a traditional mechanical engineering field, this area has seen many significant changes in recent years, most of these being related to the optimization of existing systems New systems have also evolved in response to the need for higher efficiency, size that is more compact, greater safety, and lower costs Supersonic air transport has led to several interesting innovations in this field Figure 1.16 shows a few typical systems that arise in transportation Figure 1.16(a) shows two designs for a jet engine, with hot gases being ejected from the nozzle to provide the thrust Figure 1.16(b) shows a spark ignition engine where the combustion process in the cylinder drives the piston, which moves the crankshaft and thus the wheels Figure 1.17 shows photographic views of gas turbine systems, indicating the intake, exhaust, and combustion chamber Similarly, Figure 1.18 shows sketches of engines for transportation, indicating a lightweight engine and a diesel engine that is turbocharged for boosting power Clearly, the practical systems are extremely complicated and involve intricate flow paths, combustion processes, and control mechanisms For details on these and other systems, books on thermodynamics, such as those by Van Wylen et al (1994) and by Howell and Buckius (1992), and more specialized books on various relevant topics, such as Heywood (1988), may be consulted Air Conditioning, Refrigeration, and Heating Systems Several different thermal systems are associated with this application, which is of considerable interest to us in our daily lives These include vapor compression 34 Design and Optimization of Thermal Systems FIGURE 1.16 Thermal systems for transportation (a) Thrusting systems for aircraft propulsion: Turbojet engine with and without afterburner (b) reciprocating internal combustion engine (Figure 1.16a adapted from Reynolds and Perkins, 1977, and Figure 1.16b from Moran and Shapiro, 1996) Introduction 35 FIGURE 1.17 Typical gas turbine engines for aircrafts (From AlliedSignal, Inc.) and vapor absorption cooling systems, heat pumps, ice and food freezing plants, gas, oil, and water heating systems, and refrigerators Even though this field has been around for a long time, the need for more efficient, dependable, and safe systems, at lower cost, has led to many improvements In particular, better design of the main components such as the compressor and the condenser, better control of the system, and better design of the overall system to minimize losses have resulted in reduced energy consumption and lower costs Figure 1.8 presented sketches of the vapor compression and vapor absorption systems for refrigeration In both cases, energy is removed from a given space or material due to the evaporation of the working fluid in the evaporator and heat is rejected to the ambient in the condenser The driving mechanism is the compressor in one case and the absorption process in the other In a heat pump, which operates on the same thermodynamic cycle as a refrigeration system, energy is 36 Design and Optimization of Thermal Systems LIGHTWEIGHT CONCEPT ENGINE CAM FOLLOWERS • Stamped steel w/ ceramic roller weight reduction: 24% VALVES, INTAKE & EXHAUST • Ceramic Intake weight reduction: 61% • Titanium-aluminide exhaust weight reduction: 57% VALVE SPRING RETAINERS • Polymer composite weight reduction: 84% VALVE SPRINGS • Titanium weight reduction: 57% CYLINDER HEAD • Magnesium weight reduction: 35% PISTON RINGS • 1.0mm top compression • 1.2mm 2nd compression • 2.0mm oil ring weight reduction: 35% PISTON • Magnesium forging weight reduction: 33% CYLINDER BLOCK AND GIRDLE • Magnesium with thermal spray synthectic Iron bore surface (.004'' thickness) weight reduction: 33% PISTON PIN • Ceramic weight reduction: 61% CONNECTING ROD • Forged aluminium metal matrix composite weight reduction: 33% CRANKSHAFT • Counterweights reduced due to lighter weight piston, pin and connecting rod weight reduction: 14% • Total engine weight production engine (356 lbs.) • Total engine weight as shown (306 lbs.) • Total engine weight including the additional weight reduction opportunities (292 lbs.) Additional weight reduction opportunities include magnesium or polymer composite substitutions for valve covers, front cover, oil pan, water pump housing, thermostat housing, oil pump housing, intake manifold and throttle body (a) Turbocompound diesel engine Vibration isolation (fluid coupling) Turbocharger exhaust system Power transfer to crankshaft Power turbine High speed reduction gearing (b) FIGURE 1.18 (a) A lightweight engine for an automobile (From Ford Motor Co.) (b) A turbocharged diesel engine (From Cummins Engine Co.) extracted from a colder environment and supplied to a warmer region, such as a house Photographs of practical heat pumps are shown in Figure 1.19 Though these are often treated as components, they are actually thermal systems with many interacting parts Specialized books such as those by Stoecker and Jones Introduction FIGURE 1.19 Photographs of practical heat pumps (From KIST, Korea.) 37 38 Design and Optimization of Thermal Systems (1982), Cooper (1987), and Kreider and Rabl (1994) may be consulted for details on these systems Fluid Flow Systems and Equipment These include components and fluid flow circuits such as pipe flows, hydraulics, hydrodynamics, fluidics, turbines, pumps, compressors, fans, and blowers Many of these are auxiliary subsystems to the main thermal systems and may be used for control; power transmission; cooling; and transport of mass, energy, and momentum Fluid mechanics itself is closely linked with thermal energy transport in most practical processes and fluid flow systems refer to only a subset dealing with flow circuits Figure 1.20 shows the sketches of a few typical flow distribution systems Fluid flow equipment such as pumps, fans, and blowers are extensively used in thermal systems Books on fluid mechanics, such as those by Fox and McDonald (2003), White (1994) and John and Haberman (1988), contain information on the analysis of such fluid flow systems and equipment Tank h Piping network Water treatment facility Pump Pump Lake/river FIGURE 1.20 Examples of fluid distribution systems Introduction 39 Heat Transfer Equipment Such equipment includes heat exchangers, condensers, boilers, furnaces, ovens, hot water baths, and heaters Heat transfer equipment often forms part of the various other applications mentioned here Thus, condensers and boilers may be part of a power system Similarly, furnaces may be regarded as constituents in a heat treatment system However, such equipment frequently can be designed without considering the application As mentioned earlier, in the design of a thermal system some of these items may be procured through selection rather than through design In this case, companies specializing in, say, heat exchangers, would design and manufacture these Different types of heat exchangers, such as those seen in Figure 1.5, would then be produced for selected ranges of design specifications and made available for marketing (Kays and London, 1984) Similar considerations apply for drying ovens, furnaces, heated oil baths, etc., which may be designed for a specific application or for general use Other Systems There are several other thermal systems that may not be as easily classified as was done here for some of the more common and practical systems Thus, chemical reactors and systems for experimentation, space systems, construction systems, etc also often involve thermal considerations in their design and may be treated by the techniques discussed in this book Other methods of classifying thermal systems can also be used The following approach divides these systems into three types, representing the three main stages undergone by thermal energy: Generation: Solar, geothermal, nuclear and oil-fired power systems, combustors, engines, energy conversion systems, turbines, boilers, and chemical reactors Utilization: Manufacturing, car engines, airplanes, and rockets Rejection: Heat removal, pollution, waste disposal, electronic systems, air conditioning, heat pumps, cooling towers, and radiators Even though such a classification would cover most practical systems, several systems are left out and may, again, be categorized under other systems In addition, systems such as automobiles involve all three aspects of generation, utilization, and rejection Thermal systems may also be classified by their size, by the nature and number of the constituents, by their interaction with other systems, and so on However, classification by the application of the system is probably the most useful and also the most frequently employed The preceding discussion has presented many different types of thermal systems that are of interest in a wide range of applications Clearly, different systems have different concerns, and the design specifications and requirements are also different However, they are all governed by basic considerations in heat and mass transfer, fluid flow, and thermodynamics Consequently, the basic techniques for 40 Design and Optimization of Thermal Systems design and optimization are similar, making it possible to discuss the fundamental issues and procedures involved in their design 1.4 OUTLINE AND SCOPE OF THE BOOK This book focuses on the design and optimization of thermal systems, several examples of which have been given in the preceding section The importance of thermal systems in a wide range of applications makes its essential to optimize existing systems in order to achieve the best performance or output per unit input In addition, the development of new techniques and materials demands new and improved systems to take advantage of such innovations However, the design of thermal systems is usually complicated by the complexity of the underlying mechanisms and the resulting lack of adequate information on the system for obtaining a satisfactory design Therefore, the design process often involves obtaining the relevant inputs from analysis and incorporating them into existing information on similar systems and processes to generate an acceptable design We shall first consider the basic features of the design process, highlighting the various steps that are involved in the design of a thermal system These considerations are generally common to other types of systems as well Starting with the problem statement in terms of the requirements, constraints, and other specifications, a conceptual design, which is based on creativity and existing systems, is obtained The design variables that arise in the problem are determined and varied to obtain a variety of designs, which are evaluated through analysis to determine if an acceptable design can be chosen Because the solution is not unique, a range or domain of acceptable designs is generally obtained The evaluation of the designs requires detailed information on the performance of the system Because of the complexity of most practical thermal systems, it is necessary to develop a mathematical model of the system by simplifying and idealizing the processes involved Several types of models are discussed in Chapter 3, particularly mathematical and experimental models Modeling of the system is one of the most important and creative elements in the design process because it allows relevant inputs to be generated Numerical modeling and simulation are generally needed for most practical systems, as considered in detail in Chapter Numerical simulation approximates the actual system and yields quantitative information on the behavior of the system under a wide range of design and operating conditions Therefore, the characteristics of the system can be investigated for different designs, making it possible to evaluate the design The various methods and techniques for modeling and simulation are discussed The presentation of these results in a form suitable for design is also discussed Several examples of thermal systems are then taken from different application areas in Chapter to discuss the synthesis of all these aspects to obtain a design that meets the given requirements and constraints The modeling, simulation, and design of large, practical systems are also considered Trade-offs have to be made to meet constraints due to regulations, economics, Introduction 41 safety, and other such considerations In most cases, a domain of acceptable designs is obtained, with the best or optimal design to be chosen from these If an acceptable design is not obtained, the requirements may be relaxed, new concepts considered, or the effort terminated The common problems that arise in the design process and possible approaches to avoid these are also outlined This brings us to the problem of optimization Because of growing competition in the world today, it is essential to optimize a chosen objective function such as the output per unit cost or quality per unit energy consumption The range of design variables over which acceptable designs are obtained may be quite large, making it necessary to narrow the domain to choose the best design that optimizes the cost or some other chosen variable Because economic considerations play an important role in the successful completion of the project and in the optimization effort, basic considerations in engineering economics are presented in Chapter This chapter brings out the economic evaluation of an enterprise in terms of the return on investment, costs, financing, present and future worth, and depreciation The formulation of the basic problem for optimization is discussed in Chapter 7, indicating the need for optimization and the different approaches available for thermal systems Optimization with respect to the hardware of the system, as well to the operating conditions, is discussed The next three chapters present different methods used for the optimization of thermal systems, employing examples from a variety of practical areas to illustrate the basic approaches and their limitations and advantages Among the methods considered are calculus methods, particularly the method of Lagrange multipliers, search methods, geometric programming, and linear and dynamic programming Different systems that are particularly suited to each of these methods are considered and the resulting optimal conditions derived Several examples such as those mentioned in the present chapter are employed to illustrate the strategies involved Several additional considerations are covered in the last chapter This chapter discusses knowledge-based design, which has become an important and valuable element in design methodology today The use of existing knowledge, databases, heuristic arguments, and expert systems is outlined The improvements over classical approaches for certain types of systems are presented Other considerations such as professional ethics, sources of information, and additional constraints on the design are also discussed It must be noted that the book presents all the major elements needed for the design and optimization of thermal systems However, some of these may have been covered in earlier courses at a particular college or university The instructor could then decide to avoid covering these in a given design course Examples of such topics are various aspects in economic considerations given in Chapter 6, physical modeling and dimensional analysis in Section 3.4 and solution procedures in Section 4.2 Though obviously needed for design and optimization, coverage of such topics may be curtailed or eliminated depending on the background and preparation of the class Similarly, examples of thermal systems range from simple pipe and channel flows through thermodynamic systems to more 42 Design and Optimization of Thermal Systems involved heat transfer processes Again, the instructor may choose to emphasize simpler thermodynamic and flow systems, rather than the more complicated systems that involve multidimensional heat transfer mechanisms, depending on the background of the students In many curricula, heat transfer is taught much later than thermodynamics and fluid mechanics, making it easier to consider lumped, steady, or transient, thermodynamics and fluid flow systems for design, rather than distributed ones However, wide ranges of examples, problems, and exercises are presented in this book, along with all the ingredients needed for design and optimization, to make such a choice possible on the basis of the needs and preparation of the class A Note on Problems and Examples Several examples and problems, or exercises, are given on each topic in order to strengthen the discussion and clarify the important issues involved In many cases, the problems are reasonably straightforward and build on the material presented in the book This is particularly true for examples and exercises in optimization and on other topics where a particular aspect of analysis or simulation is being demonstrated However, design involves open-ended problems and synthesis of information from different sources Many problems are given to bring out these features of the design process A unique solution is typically not obtained in these cases and the reader may have to make certain decisions, providing appropriate information or personal choice, to solve the problem The lack of particular information in a problem does not mean that it cannot be solved; instead, it implies that there are choices and inputs that must be provided by the reader to obtain different acceptable designs for the given requirements and constraints Thus, many different answers may be acceptable for a given problem Several exercises, particularly the design projects, are given with this flexibility and personal selection and input in mind Effort is also made to link the different topics and considerations that arise in the design process Thus, an acceptable design of a given thermal system in an earlier chapter may be optimized in later chapters and different techniques may be employed for the same problem to demonstrate the difference Relatively simple examples and problems are given in certain cases to illustrate the methodology However, the overall focus of the book is on the design of thermal systems, which may range from very simple systems consisting of a small number of parts to complex systems that have a large number of components and subsystems and involve many additional considerations The basic approach is similar in these two extreme circumstances, and, therefore, the discussion, treatment, and problems can be varied easily to consider different types of systems Effort was made to choose examples and problems from both traditional thermal systems as well as from new and emerging areas such as fabrication of advanced materials, cooling of electronic equipment, and new approaches in energy and environmental systems This allows the reader to see the field as vibrant and growing, with an excitement about new technologies and important Introduction 43 practical applications These examples also demonstrate the critical importance of optimizing many of these systems due to the growing need to reduce cost and energy consumption while enhancing product quality and reducing the environmental impact 1.5 SUMMARY This chapter presents the introductory material for a study of the design and optimization of thermal systems It introduces three main topics: engineering design, thermal systems and processes, and optimization In addition to providing definitions for the relevant terms, the discussion considers the basic characteristics and relevance of thermal systems and design to engineering enterprises Design, which is a creative process undertaken to solve new or existing problems, is an extremely important engineering task because it leads to new and improved processes and systems Design, which involves an open-ended solution with multiple possibilities, is contrasted against analysis, which gives rise to unique, well-defined, closed-ended results Thus, design generally involves considering many different solutions and finding an acceptable result that satisfies the given problem Synthesis brings several different analyses and types of information together, thus forming an important facet in system design In many applications, components or equipment are to be chosen from available items This is the process of selection, rather than of design, which starts from the basic concept and develops a system for a given application The focus in this book is on the design of systems and not on selection, although in several instances a particular component may be selected from those available in the market Design is also considered as part of an overall engineering enterprise The project starts with the definition of a need or opportunity and is followed by market and feasibility analyses Once these are established, engineering design is initiated with inputs from research and development The design process leads to a domain of acceptable designs from which the best or optimal design is obtained Finally, the results are communicated to other divisions of the company for fabrication, testing, and implementation Thus, design occupies a prominent position in typical engineering enterprises In most cases, optimization of the design is essential in order to obtain the best output/input ratio Processes, systems, components, and subsystems are discussed in terms of their basic features A system consists of individual constituents that interact with each other and must be considered as coupled for a study of the overall behavior Thermal systems, which are governed by the principles of heat transfer, thermodynamics, fluid mechanics, and mass transfer, arise in a wide range of engineering applications The basic characteristics of these systems are outlined, along with a few typical mathematical equations that describe them Different types of thermal systems are considered and examples are presented from many diverse areas such as manufacturing, energy, environment, electronic, aerospace, air conditioning, and transportation systems These examples serve to indicate the considerable importance of thermal systems in industry and in many practical 44 Design and Optimization of Thermal Systems applications The diversity of thermal systems and the range of concerns in these systems are also examined The need for design and optimization of these systems is clearly indicated REFERENCES Burmeister, L.C (1993) Convective Heat Transfer, 2nd ed., Wiley, New York Cengel, Y.A and Boles, M.A (2002) Thermodynamics: An Engineering Approach, 4th ed., McGraw-Hill, New York Cooper, W.B (1987) Commercial, Industrial and Institutional Refrigeration: Design, Installation and Troubleshooting, Prentice-Hall, Englewood Cliffs, NJ Dieter, G.E (2000) Engineering Design: A Materials and Processing Approach, 3rd ed., McGraw-Hill, New York Doyle, L.E., Keyser, C.A., Leach, J.L., Schrader, G.F., and Singer, M.B (1985) Manufacturing Processes and Materials for Engineers, 3rd ed., Prentice-Hall, Englewood Cliffs, NJ Duffie, J.A and Beckman, W.A (1991) Solar Energy Thermal Processes, 2nd ed., Wiley, New York Ertas, A and Jones, J.C (1996) The Engineering Design Process, 2nd ed., Wiley, New York Flemings, M.C (1974) Solidification Processing, McGraw-Hill, New York Fox, R.W and McDonald, A.T (2003) Introduction to Fluid Mechanics, 4th ed., Wiley, New York Ghosh, A and Mallik, A.K (1986) Manufacturing Science, Ellis Horwood, Chichester, U.K Heywood, J.B (1988) Internal Combustion Engineering Fundamentals, McGraw-Hill, New York Hodge, B.K (1985) Analysis and Design of Energy Systems, Prentice-Hall, Englewood Cliffs, NJ Howell, J.R and Buckius, R.O (1992) Fundamentals of Engineering Thermodynamics, 2nd ed., McGraw-Hill, New York Howell, J.R., Vliet, G.C., and Bannerot, R.B (1982) Solar Thermal Energy Systems: Analysis and Design, McGraw-Hill, New York Hsieh, J.S (1986) Solar Energy Engineering, Prentice-Hall, Englewood Cliffs, NJ Incropera, F.P and Dewitt, D.P (1990) Fundamentals of Heat and Mass Transfer, 3rd ed., Wiley, New York Incropera, F.P and Dewitt, D.P (2001) Fundamentals of Heat and Mass Transfer, 5th ed., Wiley, New York Janna, W.S (1993) Design of Fluid Thermal Systems, PWS-Kent Pub Co., Boston John, J.E.A and Haberman, W.L (1988) Introduction to Fluid Mechanics, 3rd ed., Prentice-Hall, Englewood Cliffs, NJ Kalpakjian, S and Schmid, S.R (2005) Manufacturing Engineering and Technology, 5th ed., Prentice-Hall, Upper Saddle River, NJ Kays, W.M and London, A.L (1984) Compact Heat Exchangers, McGraw-Hill, New York Kraus, A.D and Bar-Cohen, A (1983) Thermal Analysis and Control of Electronic Equipment, Hemisphere, Washington, D.C Kreider, J.F and Rabl, A (1994) Heating and Cooling of Buildings: Design for Efficiency, McGraw-Hill, New York Moran, M.J and Shapiro, H.N (1996) Fundamentals of Engineering Thermodynamics, 3rd ed., Wiley, New York Introduction 45 Moran, M.J and Shapiro, H.N (2000) Fundamentals of Engineering Thermodynamics, 4th ed., Wiley, New York Ozisik, M.N (1985) Heat Transfer: A Basic Approach, McGraw-Hill, New York Reynolds, W.C and Perkins, H.C (1977) Engineering Thermodynamics, 2nd ed., McGraw-Hill, New York Shames, I.H (1992) Mechanics of Fluids, 3rd ed., McGraw-Hill, New York Steinberg, D.S (1980) Cooling Techniques for Electronic Equipment, Wiley-Interscience, New York Stoecker, W.F (1989) Design of Thermal Systems, 3rd ed., McGraw-Hill, New York Stoecker, W.F and Jones, J.W (1982) Refrigeration and Air Conditioning, 2nd ed., McGraw-Hill, New York Suh, N.P (1990) The Principles of Design, Oxford University Press, New York Tadmor, Z and Gogos, C.G (1979) Principles of Polymer Processing, Wiley, New York Van Wylen, G.J., Sonntag, R.E., and Borgnakke, C (1994) Fundamentals of Classical Thermodynamics, 4th ed., Wiley, New York White, F.M (1994) Fluid Mechanics, 3rd ed., McGraw-Hill, New York ... Stoecker and Jones Introduction FIGURE 1. 19 Photographs of practical heat pumps (From KIST, Korea.) 37 38 Design and Optimization of Thermal Systems (19 82), Cooper (19 87), and Kreider and Rabl (19 94)... pump (b) FIGURE 1. 11 Power systems based on (a) solar energy and (b) nuclear energy (Adapted from Howell and Buckius, 19 92.) 30 Design and Optimization of Thermal Systems most of which are not... Ozisik (19 85) and Incropera and Dewitt (20 01) in heat transfer, Fox and McDonald (20 03) and Shames (19 92) in fluid mechanics, Howell and Buckius (19 92), Cengel and Boles (2002) and Moran and Shapiro